Heating device

ABSTRACT

A heating device including a pair of spaced electrodes one of which is hollow and has an outlet for a gas, an intermediate hollow electrode positioned between the principal electrodes, a control circuit including a control switch and a current sensing device connected between one of the principal electrodes and the intermediate electrode such that the current flow sensing device provides an indication of the current flowing between the principal electrodes and the intermediate electrode including a control switch and a current sensing device, an operating switch to render the control switch non-conducting and gas inlet means located between the principal electrodes and the intermediate electrode.

United States Patent 1191 New et al.

[ Mar. 4, 1975 1 HEATING DEVICE [73] Assignee: British Titan Limited,Billingham,

Teesside, England 22 Filed: Nov. 21, 1972 21 Appl. No.: 308,503

[30] Foreign Application Priority Data Dec. 9. 1971 Great Britain57155/71 [52] U.S.Cl 219/121 P,315/11l 1511 Int. CL... B23k 9/00 [58]Field of Search 219/121 P, 74, 75; 315/111; 313/731 [56] ReferencesCited UNITED STATES PATENTS 2.922.869 1/1960 Giannini et a1 219/121 P2.941.063 6/1960 Ducati et a1 219/121 P X 3.205.338 9/1965 Sunnen219/121 P 3.297.899 1/1967 Pratt et a1. 313/231 3.309.550 3/1967 Wolf eta1 219/121 P X 3.344.256 9/1967 Anderson 219/121 P 3.375.392 3/1968Brzozowski ct a1. 219/121 P X HEATED GAS OUT 3,538,378 11/1970 Kemeny etal 315/111 3,654,513 4/1972 Hammer 315/111 3.745.321 7/1973 Shapiro eta1. 219/121 P 3.760.145 9/1973 Wolf et a1 219/121 P 3.760.151 9/1973Wolf et a1. 219/383 Primary Examiner.l. V. Truhe Assistant E.raminer-G.R. Peterson Attorney, Agent, or Firm-Schuyler. Birch, Swindle-r, McKie &Beckett [5 7] ABSTRACT A heating device including a pair of spacedelectrodes one of which is hollow and has an outlet for a gas. anintermediate hollow electrode positioned between the principalelectrodes, a control circuit including a control switch and a currentsensing device connected between one of the principal electrodes and theintermediate electrode such that the current flow sensing deviceprovides an indication of the current flowing between the principalelectrodes and the intermediate electrode including a control switch anda current sensing device. an operating switch to render the controlswitch non-conducting and gas inlet means located between the principalelectrodes and the intermediate electrode.

17 Claims, 3 Drawing Figures PATENTED 41975 3.869.593

' sum 1 or 2 HEATED GAS OUT GAS IN HEATED GAS OUT GAS IN HEATING DEVICEThis invention relates to a heating device and particularly to an archeating device.

According to the present invention a heating device comprises a pair ofaxially spaced principal electrodes, one at least of which is hollow andhas an outlet for a gas, connectable to a source of high energy electriccurrent; an intermediate hollow electrode located between said pair ofprincipal electrodes and electrically insulated therefrom; a controlcircuit between one of said principal electrodes and said intermediateelectrode comprising a control switch and a current flow sensing device;an operating switch to render the control switch electricallynon-conducting; and gas inlet means located between each principalelectrode and the intermediate electrode to introduce a gas into thehollow electrodes.

Preferably, the control switch is a silicon controlled rectifier morecommonly referred to as a thyristor.

Heating devices having a pair of principal electrodes spaced apart by atleast one intermediate electrode are advantageous when used for heatinga gas in that a higher are voltage can be maintained than when theintermediate electrode is omitted and this results in an increasedtransference of energy to the gas for a constant gas flow rate andconstant arc current. Whilst it is the object of maintaining the arcbetween the principal electrodes, it has been found difficult to do thisthroughout the heating operation. There is a tendency for the arc towander between one of the principal electrodes and the intermediateelectrode.

The heating device of the present invention includes a control circuitwhich permits the intermediate electrode to be switched out of circuitwhen the arc has been established between the principal electrodes. Thecontrol circuit includes a control switch which renders the controlcircuit non-conducting and the switch must be one which can be openedquickly and one in which any tendency to arcing across the switch issubstantially eliminated. For this reason preferably the control switchis a silicon controlled rectifier commonly known as a thyristor.

To enable the control circuit to be broken the control switch,preferably a thyristor, must be opened or switched off and it isnecessary for the heating device of the present invention to include acurrent flow sensing device in this circuit to detect the flow ofcurrent between the principal electrodes and the intermediate electrode.This current flow sensing device can be a suitably positioned shuntconnected to an oscilloscope to give a visual indication when currentflow through the intermediate electrode is ceasing. Alternatively, thecurrent flow sensing device can be one which measures directly the flowof current through the thyristor from the intermediate electrode andinitiate via appropriate circuitry, opening of an operating switch toswitch off the thyristor trigger signal.

The operating switch usually is included in the trigger circuit of thethyristor and can be any suitable manual or automatic switch. Forinstance, the operating switch can be a liquid mercury switch operableby a solenoid activated either manually or automatically. For instancewhen the current flow sensing device includes an oscilloscope the manualopening of the operating switch can be affected when it is observed thatcurrent flow through the control circuit is falling. Usually, it ispreferred to switch off the operating switch and hence the triggercircuit when there is a current fiow through the control circuit of lessthan 200 milliamps. When the current next falls to zero by the arctransferring completely from the intermediateelectrode, the thyristorwill switch off.

When a thyristor is used then the control circuit should preferably alsoinclude a limiter device to prevent damage to the thyristor due to a toohigh a rate of current increase when the heating device is firstswitched on. Usually the limiter device will be a choke of suitable sizefor the electrical characteristics of the thyristor.

In addition, it has been found desirable to protect the thyristoragainst high voltage peaks by providing a voltage suppression circuitacross the thyristor in the control circuit.

These high voltage peaks are generated by the heating device itself.

Heating devices according to the invention may be used in the welding orcutting of metals or in the production of heated gas flowing at highvelocity and in the supply of heat to maintain chemical reactions.Typical chemical reactions are the oxidation of metal halides (includingsilicon) to the corresponding metal oxide or in the formation of metalcarbides by reaction between a metal halide (including silicon) and asource of carbon usually in the vapour phase.

In heating devices according to the invention, particularly those to beused in chemical reactions, the gas (es) which is heated may be an inertgas and/or a reactant. Since some erosion of the electrodes occurs whenused it is desirable in many applications that contamination of the gasby electrode material is minimised and often the effect of erosion canbe minimised by selecting electrode compositions which are compatible,for example non-colouring, with the particular product of a chemicalprocess.

Depending on the particular use of the arc heating device the principalelectrodes and the intermediate electrode may be formed from aluminium,carbon, copper, gold, iron, platinum, silver, titanium, tungsten orzirconium, or from such alloys as aluminummagnesium, copper-gold,copper-platinum-silver, copper-silver, copper-zirconium or various alloysteels.

In the heating device according to the invention the hollow principalelectrode usually is the anode but may be the cathode, if desired and isusually open at both ends. The other principal electrode axially spacedfrom the hollow principal electrode usually forms the cathode but, ifdesired can be the anode. The cathode may be a solid rod-like electrodebut preferably is also hollow but has a closed distal end. Theintermediate electrode is also hollow and open at both ends to permitthe flow of gas and the arc to pass through the electrode. Usually thehollow electrode is at earth potential.

If desired the hollow principal electrode can have a bore of increasinginternal diameter from the end adjacent the intermediate electrode. Thebore can be said to be conical and may, have a cone angle of l to 30preferably 2 to 20 It has been found desirable to position a field coilaround the cathode to generate a magnetic field to rotate the are withinthe cathode to reduce erosion of the electrodes.

When the outer electrode has a conical bore then it has been foundadvantageous to apply a magnetic field to the are from a field coilaround this electrode. An increase in gas enthalpy has been observed.This magnetic field is usually applied in addition to that applied tothe other principal electrode. A suitable range of peak magnetic fluxdensity is 0.0l to 4.0 tesla with a preferred range of 0.05 to 1.0tesla.

Usually, it is necessary to provide cooling of the electrode during useand to enable this to be done the elctrodes will usually either besurrounded by cooling chambers or have channels in their walls to permitcirculation of a cooling fluid. Preferably, the cooling fluid is waterof a controlled quantity. Preferably the water has a heat flux capacityof at least 100 watts. The use of such cooling water is described andclaimed in our co-pending patent application No. 50521/71. Preferablythe water contains a nucleating agent for bubble formation and suchagents are an alcohol, an organic acid or an inorganic acid salt of ametal such as aluminturn.

The heating device is provided with suitable inlets located between theintermediate electrode and the principal electrodes for a gas to beheated by passage through the device. If desired, these inlets can be soshaped and/or positioned to provide a tangential and/or helical gas flowthrough the device. If multiple gas injection points are employed thenthe gas must be introduced in the same manner and sense, e.g. tangentialand clockwise between the electrodes.

The heating device can be operated by any suitable power source and thehigh energy electric current required to strike and maintain the arc canbe high voltage-low current, low voltage-high current or high currentand high voltage. For instance, the device can be operated in the rangel volts to 20,000 volts and from amps. to 5,000 amps.

The heating device of the present invention is particularly useful inthe manufacture of titanium dioxide by the vapour phase oxidationprocess. In this process, a titanium tetrahalide, e.g. the tetrachlorideis oxidised in the vapour phase to produce pigmentary titanium dioxide.The oxidation process is carried out at an elevated temperature and itis necessary to preheat one or both of the reactants or an inert gas toa sufficiently high temperature to initiate the reaction. The heatingdevice of the present invention is preferably used to heat oxygen or aninert gas in the process for the production of pigmentary titaniumdioxide.

One form of heating device constructed in accordance with the inventionwill now be described by way of example only with reference to theaccompanying drawings, in which FIG. 1 is a longitudinal sectional viewthrough the device.

FIG. 2 is a diagrammatic view of the electric circuit.

FIG. 3 is a sectional view similar to FIG. 1 illustrating a modifiedform of the device.

As shown in FIG. 1 the device includes a hollow front electrode 1 whichis the anode and axially spaced .therefrom is the cathode 2. Anintermediate hollow electrode 3 is mounted between the anode 1 andcathode 2 and is electrically insulated therefrom by means of insulatingmembers 4 and 5.

The anode 1 is shaped to provide an annular channel 6 through which thegas to be heated is passed via inlet pipes 7 and 8 and thence to theinterior 9 of the hollow anode. The cathode 2 is also shaped to providea similar annular channel 10 supplied by inlet pipes 11, 12.

Each electrode 1 and 2 is surrounded by a chamber through which coolingwater can be passed but this is not'shown on the drawings.

The control circuit for the device is shown in FIG. 2. The anode 1 andintermediate electrode are electrically connected via a thyristor 13provided with a water cooled copper block 14 for cooling. Cooling wateris circulated through the block 14 via inlet 15 and outlet 16. A choke17 and current flow sensing device 18 incorporating an oscilloscope areconnected in the control circuit. The oscilloscope is connected to givea vi- :sual indication of the current flowing through the controlcircuit. The choke 17 prevents too rapid an increase in current throughthe thyristor and thereby reduces the risk of inadvertent damage to thethyristor 13 choke 17 is connected by a lead 29 to intermediateelectrode 3.

Connected across the thyristor 13 is a protection circuit 19 to protectthe thyristor 13 against high voltage peaks and this protection circuitincludes a resistor 20 in series with a resistor 21 and a condenser 22connected in parallel.

The trigger circuit 23 for the thyristor 13 includes a a supply 24 ofstabilised, smoothed direct current, a liquid mercury filled switch 25and a resistor 26 to ensure that there is no potential difference acrossthe trigger circuit 23 at the thyristor 13 when the switch 25 is open.

A source of high energy electric current is connected across the anode 1and cathode 2 via leads 27 and 28 and the anode 1 is earthed.

In operation the trigger circuit 23 is energised by closing switch 25and a high energy direct electric current is fed across the anode l andcathode 2 via leads 27 and 28. An arc is struck initially between thecathode 2 and the intermediate electrode 3 since the thyristor isconducting.

Gas is passed into the chamber 10 and into the interior of theintermediate electrode and the arc is transferred for a high proportionof the time on to the anode 1. Thistransference is effected byadjustment of the current and flow of gas. The current flow sensingdevice 18 indicates via the oscilloscope when the arc is transferredsince current flow through the control circuit falls. When this currentflow has fallen to, say, less than 200 milliamps then the switch 25 isopened to open the trigger circuit. When the trigger circuit is switchedoff the thyristor 13 is rendered potentially non-conducting and when theelectrical flow to the intermediate electrode next falls to zero the arcis stabilised between the anode 1 and cathode 2.

Gas is then passed through inlets 7 and 8 and this gas, together withthat admitted through inlets 11,12 is heated by the arc and passed outof the device through the anode 1.

FIG. 3 shows a device which is in many respect similar to that shown inFIG. 1 with like parts being numbered similarly. The hollow frontelectrode 30 in this device has an increasing internal diameter towardsthe outer end and is provided with a field coil 31 surrounding theelectrode. The additional field coil 32 surrounds the cathode 2. Thedevice is connected to a control circuit in a manner similar to thatshown in FIG. 2.

In the heating device of the present invention, more than oneintermediate electrode can be present and in such a device eachintermediate electrode will form part of a control circuit so thatprogressive switching of the are from one intermediate electrode to thenext and finally on to the principal electrodes takes place.

The invention is also illustrated in the folllowing Examples.

EXAMPLE l A heating device with a control circuit as show in FIGS. 1 and2 was set up. The anode l was at earth potential and had an internalbore of 1.5 inches. The intermediate electrode 3 had an internal bore of1.25 inches and was 6 inches long. A magnetic field of 0.25 teslamaximum was applied to the cathode.

The circuits were energised and gas passed through inlets l1 and 12. Theare was struck and when the gas flow rate and current had been increasedto transfer the arc for the majority of the time on to the anode, theswitch 25 was opened and the arc was stabilised between the anode andcathode. Gas was then also passed into the device through inlets 7 and8. The gas was oxygen.

The total gas flow was 12,000 standard cubic feet per hour. The arccurrent was 300 amps. and the arc voltage was l,850 volts.

In a device without the intermediate electrode with the same gas flowand arc current, the arc voltage was 1,525 volts.

The device of the present invention clearly operated at a high voltageand there was a corresponding proportionate increase in average gasenthalpy.

EXAMPLE 2 The method of example 1 was repeated except that the outerelectrode 1 was made the cathode and no magnetic field was applied tothe cathode. An arc voltage of 1,900 volts was obtained on energizingthe circuits but in a similar device without the intermediate electrodean arc voltage of only 1,600 volts was obtained.

EXAMPLE 3 The experiment described in Example 1 was repeated except thatthe anode l was provided with an outer field coil to apply a peakmagnetic field of 0.15 tesla. On energizing the device as described inExample 1, with the intermediate electrode in position an arc voltage of1,800 volts was obtained.

In a similar device without the intermediate electrode an arc voltage of1,525 volts was obtained.

These results show that by applying a magnetic field to a non-conicalelectrode no significant effect is observed on the performance of thedevice.

EXAMPLE 4 In this example the apparatus described in Example 1 wasmodified so that the anode l was used at earth potential and had aninternal diameter of 0.75 inch at its narrowest part and had a conicalinner bore with a cone angle of 2.5. The intermediate electrode had aninternal bore of 0.375 inches and was 5 inches long. A magnetic field ofpeak strength 0.25 tesla was applied to the cathode. Total gas flow was750 standard cubic feet per hour.

The are current was 80 amps and an arc voltage of 600 volts was obtainedwhen the circuits were energized.

When the intermediate electrode was omitted in a similar device it hadan arc voltage of 525 volts.

EXAMPLE 5 Example 4 was repeated except that the conical outer electrodewas provided with a magnetic field of peak strength 0.15 tesla. When soused the device produced an arc voltage of 800 volts which wasappreciably greater than that when no magnetic field was applied to theanode.

EXAMPLE 6 The experiment described in Example 4 was repeated except thata magnetic field of peak strength 0.325 tesla was applied to the anodeand the gas flow rate was 1,150 standard cubic feet per hour with an arccurrent of amps. On energizing the circuits an arc voltage of 990 voltswas obtained when the intermediate electrode was in position.

In a similar device without an intermediate electrode an arc voltage of700 volts was obtained.

What is claimed is:

1. A heating device comprising a pair of axially spaced principalelectrodes, at least one of which is hollow and has an outlet for a gas;said principal electrodes being connected to a high energy electriccurrent; an intermediate hollow electrode located between said pair ofprincipal electrodes and electrically insulated therefrom; a controlcircuit comprising a control switch and a current flow sensing deviceconnected between one of said principal electrodes and said intermediateelectrode such that said current flow sensing device senses the currentflowing between one of said principal electrodes and said intermediateelectrode; an operating switch electrically connected to said controlswitch and operable to render said control switch nonconductive when thecurrent between one of said principal electrodes and said intermediateelectrode is sensed to have fallen below a predetermined value; gasinlet means disposed between each of said principal electrodes and saidintermediate electrodes for introducing gas into said hollowintermediate and said hollow principal electrodes.

2. A heating device according to claim 1 in which the control switch isa silicon controlled rectifier.

3. A heating device according to claim 1 in which the operating switchis a liquid mercury switch operable by a solenoid.

4. A heating device according to claim 2 in which the control circuitincludes a limiter device to prevent damage to the silicon controlledrectifier, as a result of too high a rate of current increase uponactivation of the device.

' 5. A heating device according to claim 4 in which the limiter deviceis a choke of suitable size for the electrical characteristics of thesilicon controlled rectifier.

6. A heating device according to claim 2 in which a voltage suppressioncircuit is provided across the silicon controlled rectifier in thecontrol circuit.

7. A heating device according to claim I in which said hollow principalelectrode is open at both ends.

8. A heating device according to claim 7 in which said hollow principalelectrode has a bore of increasing internal diameter from the endadjacent the intermediate electrode.

9. A heating device according to claim 8 in which the inner surface ofsaid hollow electrode is conical and has a cone angle of from 1 to 30.

10. A heating device according to claim 9 in which said cone angle isfrom 2 to 11. A heating device according to claim 8 in which a fieldcoil is positioned around said hollow principal electrode to provide apeak magnetic flux density of from 0.01 to 4.0 tesla when in operation.

12. A heating device according to claim 11 in which said field coil issuch as to provide a peak magnetic flux density of 0.05 to 1.0 teslawhen in operation.

13. A heating device according to claim 1 in which the other principalelectrode is hollow and has a closed distal end opposite the hollowintermediate electrode.

14. A heating device according to claim 13 in which said other principalelectrode has a field coil around amps to 5,000 amps.

1. A heating device comprising a pair of axially spaced principal electrodes, at least one of which is hollow and has an outlet for a gas; said principal electrodes being connected to a high energy electric current; an intermediate hollow electrode located between said pair of principal electrodes and electrically insulated therefrom; a control circuit comprising a control switch and a current flow sensing device connected between one of said principal electrodes and said intermediate electrode such that said current flow sensing device senses the current flowing between one of said principal electrodes and said intermediate electrode; an operating switch electrically connected to said control switch anD operable to render said control switch nonconductive when the current between one of said principal electrodes and said intermediate electrode is sensed to have fallen below a predetermined value; gas inlet means disposed between each of said principal electrodes and said intermediate electrodes for introducing gas into said hollow intermediate and said hollow principal electrodes.
 2. A heating device according to claim 1 in which the control switch is a silicon controlled rectifier.
 3. A heating device according to claim 1 in which the operating switch is a liquid mercury switch operable by a solenoid.
 4. A heating device according to claim 2 in which the control circuit includes a limiter device to prevent damage to the silicon controlled rectifier, as a result of too high a rate of current increase upon activation of the device.
 5. A heating device according to claim 4 in which the limiter device is a choke of suitable size for the electrical characteristics of the silicon controlled rectifier.
 6. A heating device according to claim 2 in which a voltage suppression circuit is provided across the silicon controlled rectifier in the control circuit.
 7. A heating device according to claim 1 in which said hollow principal electrode is open at both ends.
 8. A heating device according to claim 7 in which said hollow principal electrode has a bore of increasing internal diameter from the end adjacent the intermediate electrode.
 9. A heating device according to claim 8 in which the inner surface of said hollow electrode is conical and has a cone angle of from 1* to 30*.
 10. A heating device according to claim 9 in which said cone angle is from 2* to 20*.
 11. A heating device according to claim 8 in which a field coil is positioned around said hollow principal electrode to provide a peak magnetic flux density of from 0.01 to 4.0 tesla when in operation.
 12. A heating device according to claim 11 in which said field coil is such as to provide a peak magnetic flux density of 0.05 to 1.0 tesla when in operation.
 13. A heating device according to claim 1 in which the other principal electrode is hollow and has a closed distal end opposite the hollow intermediate electrode.
 14. A heating device according to claim 13 in which said other principal electrode has a field coil around the electrode to generate a magnetic field within the electrode when in use.
 15. A heating device according to claim 1 wherein means are provided to cool the electrodes when in use.
 16. A heating device according to claim 1 in which the gas inlet means are shaped and/or positioned to provide a tangential or helical gas flow through the device when in use.
 17. A heating device according to claim 1 having a power source sufficient to provide a high energy electric current of from 100 to 20,000 volts and from 10 amps to 5,000 amps. 